This study aimed to determine the effectiveness of a professional development program on the use of a STEM-based 5E Inquiry Learning Model in enhancing the learning of STEM-based Inquiry Learning Model among 78 in- and pre-service science teachers. The topic electric circuit was used as the context of the study. Given the aim and the exploratory nature of this study, a triangulation mixed-methods design was employed: One-group pretest-posttest design which involved a single group that was pre-tested, exposed to the STEM-based 5E Inquiry Learning Model, and post-tested. This was triangulated by the qualitative responses from the participants' reflections. The analysis of the quantitative dataset using the paired samples t-test indicated that the post-test mean score was statistically significantly higher than the pre-test mean score and that the effect size obtained was educationally significant. Further analyses by gender and participants' teacher status, indicated a non-significant difference, giving credence that this result was truly the outcome of the intervention (treatment) effect instead of any disguised effect. The analysis of qualitative data through a recursive process resulted in the crystallization of three overarching key themes: Enhanced knowledge on the content used, empowered pedagogical skills on STEM-based Inquiry Learning, and heightened classroom management skill on promoting collaboration in the classroom.